In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been, and continues to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such a high device packing density, smaller features sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as the corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. With regard to semiconductor fabrication, a silicon slice (e.g., a wafer) is coated uniformly with a radiation-sensitive film of material (e.g., a photoresist). The coated substrate can be baked to evaporate solvents in the photoresist composition and to fix the photoresist coating onto the substrate. An exposing source (e.g., light, x-rays, an electron beam) can then be utilized to illuminate selected areas of the surface of the film through an intervening master template (e.g., a mask or reticle) to affect the transfer of a pattern formed within the template onto the wafer. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image from the intervening master template is projected onto the photoresist coating, it is indelibly formed therein.
Light projected onto the photoresist layer during photolithography changes properties (e.g., solubility) of the layer of material such that different portions thereof (e.g., the illuminated or un-illuminated portions, depending upon the type of photoresist utilized) can be manipulated in subsequent processing steps. For example, regions of a negative photoresist become insoluble when illuminated by an exposure source such that the application of a solvent to the photoresist during a subsequent development stage removes only non-illuminated regions of the photoresist. The pattern formed in the negative photoresist layer is, thus, the negative of the pattern defined by opaque regions of the template. By contrast, in a positive photoresist, illuminated regions of the photoresist become soluble and arc removed via application of a solvent during development. Thus, the pattern formed in the positive photoresist is a positive image of opaque regions on the template.
Projection lithography is a powerful and important tool for integrated circuit processing. However, as feature sizes continue to decrease, optical systems are approaching their limits caused by the wavelengths of the optical radiation being utilized. A recognized way of further reducing feature sizes is to lithographically image them with radiation of a shorter wavelength. Extreme ultraviolet (EUV) or “soft” x-rays, which have wavelengths within a range of about 30 to 700 Angstroms (i.e., about 3 to 70 nm), can, for example, be considered as an alternative radiation source in photolithography processing in an effort to achieve desired feature sizes.
EUV lithography may be carried-out, for example, in an EUV lithography system, such as that illustrated in prior art FIG. 1. The prior art system 100 depicted in FIG. 1 is designed to delineate a latent image (not shown) of a desired circuit pattern (e.g., having feature dimensions of 0.18 μm and less) onto a wafer 102 by illuminating a reflective mask 104 with EUV radiation and having at least a portion of that radiation reflected onto the wafer (e.g., via a system of mirrors). The portion of the radiation reflected onto the wafer 102 corresponds to the desired circuit pattern that is to be transferred onto the wafer 102. FIG. 1 is a simplified schematic representation of such a system wherein certain components are not specifically shown.
By way of example, EUV radiation 106 having a wavelength of 3 nm to 70 nm, for example, can be generated from a light source 108, such as a synchrotron or a laser plasma source that can include optical filtering elements 110 and a reflective condenser 112. The condenser and filtering elements can collect the EUV radiation and project one or more beams 114 onto the reflective mask 104 through a slit (not shown), for example, having a particular width and length. The reflective mask 104 absorbs some of the EUV radiation 116 and reflects other portions of the EUV radiation 118 corresponding to one or more features or circuit patterns formed on the mask. The reflective system can include, for example, a series of high precision mirrors 120 (e.g., concave and/or convex mirrors) which can cause the radiation to converge and/or diverge in projecting a demagnified or reduced image of the pattern(s) to be transferred onto the resist-coated substrate 102. Typically, the reflective mask 104 and wafer 102 are mounted to stages (not shown) such that a scanner can move the mask 104 and the wafer 102 at respective orientations and speeds relative to one another (e.g., in a step and scan fashion) to effect a desired mask-to-image reduction and to facilitate pattern transfers onto one or more different die on the wafer.
The mask 104 of prior art FIG. 1 is an important component in the EUV lithography system 100. Unlike conventional UV lithography systems which predominately use refractive optics, many EUV lithography systems, such as the system 100 depicted in prior art FIG. 1, utilize reflective optics. The mask 104 is thus a reflective mask that reflects at least some incident EUV radiation to transfer a pattern onto a wafer during a semiconductor fabrication process, as opposed to allowing some of the radiation to pass through selected portions of the mask.